John 8: 27-38, Abraham's descendants

Jesus has been speaking to the Pharisees, claiming God as his Father. In the most recent teaching, he has spoken of the One who sent him.

John 8: 27, Still confused

They did not understand that he was telling them about his Father.

Despite the allusions to God, the Pharisees wonder if someone else has sent Jesus to preach to them.

John 8: 28-32, The Son of Man and truth

So Jesus said, “When you have lifted up the Son of Man, then you will know that I am he and that I do nothing on my own but speak just what the Father has taught me.
29 The one who sent me is with me; he has not left me alone, for I always do what pleases him.” 

30 Even as he spoke, many believed in him.
31 To the Jews who had believed him, Jesus said, “If you hold to my teaching, you are really my disciples. 
32 Then you will know the truth, and the truth will set you free.”

Jesus represents God and acts as God in human form.  He is the "Son of Man" (that is, the Messiah).

The gospel writer reports that in these conversations, Jesus does pick up believers and he promises those believers that the truth (about him) sets them free (from darkness.)

John 8: 33-38, God's true family
They answered him, “We are Abraham’s descendants and have never been slaves of anyone. How can you say that we shall be set free?”

34 Jesus replied, “Very truly I tell you, everyone who sins is a slave to sin. 
35 Now a slave has no permanent place in the family, but a son belongs to it forever. 
36 So if the Son sets you free, you will be free indeed. 
37 I know that you are Abraham’s descendants. Yet you are looking for a way to kill me, because you have no room for my word. 
38 I am telling you what I have seen in the Father’s presence, and you are doing what you have heard from your father.”

Although currently under the Roman thumb, and previously captives of Babylon and then controlled by Ptolemaic rulers, the Pharisees maintain a fiction that they have never been slaves of anyone. This lie is irrelevant, for they are "slaves to sin", under the control of their own selfish passions. Later, the former Pharisee, Paul, will elaborate on this in Romans 7: 14-25.  (Paul will also describe the privileges of being a son/daughter, no longer a slave, in God's family in Galatians 4: 1-6.)